The present invention relates in general to transformers and, more particularly, to the control of magnetizing current in inverter-fed transformers.
Switch-mode converters employing pulse width modulation (PWM) are widely used for dc-dc and dc-ac applications. This type of converter can be used in a dc-ac inverter configuration to produce a power-frequency sinusoidal voltage that is applied to an output transformer. Because the inverter may produce an unwanted small direct voltage across the transformer in addition to the power-frequency sinusoid, dc saturation of the transformer core may occur. A technique that might be considered to compensate for this effect is direct measurement of the inverter output voltage with a proportional-integral (PI) controller used to remove the dc component. This technique can remove the dc component of voltage over time, but can leave the transformer with a surplus or deficit of volt-seconds, resulting in persistent dc current. For this reason, prevention of dc saturation is conventionally accomplished by measurement of the transformer primary current, from which the dc component is extracted by analog and/or digital filtering techniques. Measurement of the transformer primary current is conventionally accomplished by insertion of a low-ohmic sense resistor in series with the transformer primary, or by routing the transformer primary through a dc measurement transducer, which may be a Hall effect or other device.
The conventional techniques have several drawbacks. A first drawback is the excessive rating requirement for the current measuring device in that the device must be rated for the full primary current. Another drawback is the slow response times that are the result of filtering time delays. In other words, analog filtering to extract the dc component from a power frequency component (e.g., 60 Hz) must necessarily employ large time constants. Digital (rolling average) filtering can improve this, provided that arrangements are made to ensure that the filtering algorithm is phase-locked to the power-frequency output voltage. Yet another drawback is the sensitivity to dc drift in the measuring electronics. Any technique that attempts to measure the dc component of the transformer primary current introduces the difficulty of distinguishing between real dc current and dc drift in the electronics due to temperature and/or other effects. For this reason, the conventional techniques may use a sense resistor in series with the transformer primary. Use of a sense resistor eliminates the dc drift that is a characteristic of Hall effect devices, but the sense resistor introduces additional losses into the circuit. For high-current applications, the losses introduced by the sense resistor can become prohibitive. Furthermore, this technique does not eliminate other sources of dc drift in the analog electronics used to amplify the current signal.
Thus, there is a need to control the magnetizing current and compensate for dc saturation of the transformer core with faster response time, low losses, and with immunity to dc drift in the measuring electronics.
The present invention is directed to the use of differential measurement of the transformer magnetizing current and a delta-modulation technique to provide compensation for dc saturation of the transformer core, with faster response times, low losses, and with immunity to dc drift in the measuring electronics.
According to embodiments of the invention, systems and methods of controlling a magnetizing current in a transformer having an output voltage and a peak primary voltage are provided, and comprise determining a positive peak value of the magnetizing current during a half-cycle in which the output voltage transitions from a positive maximum to a negative maximum; storing the positive peak value and the negative peak value; determining a second positive peak value of the magnetizing current during a second half-cycle in which the output voltage transitions from the positive maximum to the negative maximum; determining an onset of core saturation in a positive direction based on the positive peak value of the magnetizing current and the second positive peak value of the magnetizing current; determining a control action based on the onset of core saturation; and implementing the control action to control the magnetizing current in the transformer.
According to aspects of the invention, determining an onset of core saturation in a positive direction comprises comparing the difference between the positive peak value of the magnetizing current and the second positive peak value of the magnetizing current with a predetermined threshold value.
According to further aspects of the invention, a windowing technique is used whereby a negative peak value of the magnetizing current is determined during a half-cycle in which the output voltage transitions from the negative maximum to the positive maximum; the negative peak value is stored; a second negative peak value of the magnetizing current is determined during a second half-cycle in which the output voltage transitions from the negative maximum to the positive maximum; and an onset of core saturation in a negative direction is determined based on the negative peak value of the magnetizing current and the second negative peak value of the magnetizing current.
According to further aspects of the invention, determining the control action comprises detection of the onset of core saturation in the positive direction and the onset of core saturation in the negative direction.
Another embodiment of the present invention is directed to a method of controlling the output voltage of a transformer having a transformer core, comprising detecting a dc saturation in the transformer core; determining a transformer magnetizing current; receiving a signal proportional to the transformer magnetizing current; and adjusting the output voltage responsive to the signal.
According to aspects of the invention, the transformer magnetizing current comprises detecting a current using a dc sensor, and the signal is generated proportional to the transformer magnetizing current based on the detected current. Moreover, a dc component of the output voltage is adjusted to compensate for the dc saturation of the transformer.
According to further aspects of the invention, a continuous comparison technique is used to detect the dc saturation in the transformer core and comprises searching for positive and negative peak values of the magnetizing current, updating the positive and negative peak values once per power frequency cycle, storing the positive and negative peak values, and comparing the positive and negative peak values from the current cycle to the positive and negative peak values in the previous cycle.
A further aspect of the present invention is directed to a method of extracting a magnetizing current in a transformer having a plurality of windings, the transformer having a primary and a secondary and providing fluxes due to a transformer primary current and a transformer secondary current. The method comprises disposing the windings through a current sensor in such a way that the fluxes due to the transformer primary current and the secondary current cancel, thereby providing the magnetizing current as the output of the current sensor.